Expansion furnace wall



J1me 193i. 0. NYGAARD EXPANSION FURNACE WALL Filed Oct. 30, 1926 3 Sheets-Sheet l INVENTOR.

A TTORN June 2, 1931, O NYGAARD EXPANSION FURNACE WALL Filed Oct. 30, 1926 3 Sheets-Sheet 2 E 5 R w C \\\m 6 v 1 /s W vy 7 M Q E m u RI |l|l L w n m v LL W 9 q. E E m m A M H u Q a E a 4' INVENTOR.

Jum 1931- o. NYGAARD L87,68

EXPANSION FURNACE WALL Filed Oct. 30, 1926 3 Sheets-Sheet 5 5 Games:

Patented iJuneZ, 193i osena uremia,- or sevens, mssacmrsnr'rs mansion rnaiucn warn.

Application filed October 3, 192 6. smaim. 145,270.

This invention relates to wall structures and more particularly to furnace walls having' a construction that permits an adequate free expansion of the blocks of which it is 5 built under the intense but variable heat l within the furnace.--l he invention is espe- 'ci'al-ly applicable tofurnaces employing'a.

forced air feed.

The primary object of the p'resentinvem tion is to improve the construction of prior known expansion furnace walls by providing for. either a successive or a-simultaneous expansion of different areas of the .wall at any time that expansion is requisite to prei5 serve the wall structure,'thus giving a longer life to the wall. I A further objegtiof the invention lS l}O simplify the construction of expansion furnace walls.

. 'Accordingly an important feature of the invention "consists in the construction and relative arrangement of the blocks of which; the heat resisting wall is built whereby each side or surface thereof is divided into relativelysmall units or areas each havin its 'own .individual expansion joints. Pre era bly the arrangement i such that these wall units overlap each other or, stated in another way, the expansion joints provided primarily for. one unit also function to contributeto, .or aid in, the expansion of all at a relatively low figure. 1

To the accomplishment of these objects and such others "as may hereinafter appear, as

will readilykbe understoodby those" skilled in the art from the following description, the invention comprises the novel constructional features, arrangement and combination of parts for-furnace'walls hereinafter described in their preferred form andthen particu: larly pointed out inthe appended claims.

1 The various features of the invention will surrounding units. An equally important best be understood'from a description of he best form at present known to me, illustrated in the accompanying drawings, in'which:

Figuretl isa view, in perspective, of a-portion of a masonry Wall fo furnaces lined 5 with an inner facing of hi h grade refractory materialhavingthe novel characteristics p of the present invention;

:Fig. 2 is a view, in sectional plan, of a portion of the wall shown in Fig. 1, on the line 1) Z2ofFig.1;

Fig. 3 is a view in which each .of the three different shapes, for the blocks from which thewallis built, is shown separately in per- 1 spective. In this figure two 0 the shapes are shown in each of the two difi'erent positions in which they are placed when building them into different portions of the wall r Fig. 4 is'a view, in front elevation, of a singleexpansion area or unit illustratin how' thethree shapes are duplicated and com ined to produce said unit; I i

Fig. 5 is a view, in'-vertical sectional eleva-' tion, of a modified constructionof key-block ing locks in the wall; f J

Fig. 6. is a view, in frontelevation, of a ortionof the type of expansion wall shown in Fig. 1, but modified to provide continubus lateral air-spaces;

Fig; 7 is a view, in section,.on the line 66 of Fig. 6 illustrating the continuity of a lateral air-space; I 6 r Fig. 8 is a view, in sectional elevation on the line 8 -8 of :the modified wall of Fig 6 1 showing a'lateral air-space; and r Fig.9.is a view, in vertical sectionalelevation, of a further modification of themean-s for yieldingly supporting the key-block of each expansion joint. 7

'In the embodiment of the invention illustrated by .Figs. 1 to4 inclusive 10 indicates the on r masonry ,wall of the furnace which for 'roviding forexpansio'nofthe'surroundis of equal thickness or depth throughout; its I height except-that it is provided at certain intervals on'its inner vface with projectin are so arranged as to present repeated groups 1 except the central block, also forms a part of some adjoining unit. The central block of. each group of nine makes ample provision forwhich, however, are not definitely separable except in. the case of certain centrally arrangedsupports,buteach such complete group comprises the requisite 'number of supporting shelves for maintaining in position in the fire wall a certain definite number or' group of blocks forming what may be termed an expansion unit. It will be understood from the drawing that the fire wall, in its preferred form, is composed of building blocks all of which present a rectangular face at the inner or heat resisting surface of the wall. This fact should be borne in mind in following the detailed description of the-fire wall now to :be given.

Each complete expansion unit is composed of nine building blocks which form a rec tangle (see Fig. 4)' but theseunits are so built into the fire wall, or the wall is so designed, that each block forming each unit,

expansion of all the blocksof the expansion unit of which it is the center, but because of the fact that each expansion unit extends into the structure and forms a part of those that are adjacent it, the expansion joints'provided by these central blocksdo-duty for more than one unit as will presently be explained.

The general. scheme of the fire wall constrliction will be recognized by comparing the single expansion-unit illustrated'lby Fig. 4 with the larger portion of a wall built from a number of these units, illustrated by Fig. 1.

- The construction of a singe expansion unit will first be described and then the building of a completefire wall-will be described. Refer-ring first, then, to Fig. 4:

The central block of an expansion unit may be termed the expansion block and is designated by E. This block has a rectangular heat resisting end or face that is somewhat smaller in area than the inner ends of the eight blocks which surround it; In practice the dimensions of the innerend of expansion block E may conveniently be about less on each side than those of the surrounding blocks. When these eight surrounding blocks are formed into a hollow rectangle, and the block E is held centrally in the central opening, an air space or expansion-joint 12 will be produced of rectangular formation and completely separating all eight surrounding blocks from the block E. 'The manner of holdin each block E centrally as just describe will be explained when the wallconthe other, designated by G, is grooved.- on twosides of its head nearits inner end.

This mannerof binding the blocks to gether to form a unitary structure permits a sliding movement of one block on another in two directions and efiectua1l 'accomn1odates the wall expansion. bothorizont'ally and vertically. The blocks G are provided with grooves 14 and won two adjoining sides and the other two sides are plane surfaces. Likewise the blocks T are provided with'tongues 18 and 20 on only twolsides but these tongues (see Fig; 3) are on the sides corresponding to the grooved sides of the blocks G. When built into the wall the blocks are always placedabo ve andbelow a block E, those above having the grooves 14 and 16 at the top and left, hence called GL blocks, and those below having the grooves 14; and '16- at the bottom and. r1ght,- hence called GR blocks. The blocks tofore stated the G ,blocks are all alike and the T blocks are all'alike, The apparently differing positions ofthe tongues and grooves results from placing half of the blocks in a normal or upright position and the otherhalf in a reverse position. An ins .ection of Fig 4illustrates how four tongue and four grooved blocks may be fitted together about a; block E to form an expansion unit.

Before describing the manner of building the fire wall a brief description will be given of the characteristic features of the three block shapes E, G and T, of which the entire fire wall is built.

' Expansion block E (Fig. 3) consists of a rectangular head 22 and a. rectangular base 24 of larger proportions joined b a beveled surface 23 at each of the four si e's. It has been foundefiective in practice to form the bevel 23* at a 45 angle to the plane of the head and base but this angle may be varied within: reasonable limits withourt detriment-I Grooved blocks GR and GL (Fi 3) are exactly alike and this nomenclature as been. given because the blockG is placed-in reverse position when below the block E. This is indicated in Fig. 4 bythe inverted lettering. Block G slightly larger than'the head of. block E both as to surface dimensions and depth, and a body. 28 of less height than the head joined to the head bya bevel 27 on its under side only when inupri'ghtposition (see GL block Fig. 3).. In the corner of thetop. face 0 h'as a rectangular head-26',

GL, Fig; 3', opposite the right hand end of thebevel, there is a recess-30 having the eometrical shape r uisite to receive the W, two-way beveled ase-corner of a block onFig. 4 by the inverted lettering. Block V E. The block-locking1 grooves Hand 16' are at the to and-left w en block G is upright and at t e bottom and right when it is invertcd (see GR Fi 3).

Tongued blocks "FL and TR (Fig. 3 also are exactly alike but TR, situated always at the right of a block E isin a reversed position as compared to TL. This is indicated T has a rectangular head 32 of the same dimensions as the head 26. of block G and a bodv- 34 of less width than the head joined to the headby a bevel 33 on its right side only when in upright position, (see TL block, Fig. 3'). In thecorner of the top face opposite the up rend of the bevel there is a recess 36, li e the recess 30 of the block G,

for receiving the SE,.two-way beveled corner of a-block E. I The block-locking tongues 18 and '20 are at the bottom and left when the block T'is upright and at the top and ii ht when it is inverted (see TR, Fig. 3).-

T block T is further distin ished from the block G by a rabbet 38 in t e under face of its body 34 (when upright) of suchdimension's that the rear face of said body is the same height as the vertical-dimension of therear face of the body 28 of block G. In practice this height is equal to two courses of the brick forming the outer masonry wall 10. It-may ,be stated here that the vertical dimension ofthe rectangular base 24 of block E is slightly,.less than the hei ht of five courses of the masonry wall bric and that the'vertical dimension of the heads of both blocks G and T is equal to the height of three courses of .the masonry wall brick (see Fi 1). i a The design of the inner fire wall is very clearly disclosed by Fig. 1 from which it will be seen that in each course thereof every fifth block is an expansion blockE thus. roviding four contacting buildingblocks and '1,

horizontally between each two expansion 1 blocks E. A like construction is found also in each vertical row of blocks and is obtained by offsetting the expansion blocks, in each successive course, by two positions, which provides for four contacting buildingblocks G and T vertically between each two expansionblocks E. It will be observed that the design of each set of five courses is repeated but that the blocks G and T that surround 1 relation of expansion units is ahi hlyim a first or fifth course expansion block tothus form an expansion unit are partly in one set and partly in another. In fact this interp'ortant characteristic of the fire w Each marginal block G or T is always a"pa-rt.of'

two expansion units, shown in detail by Fig. 4.

Considering now the construction of a complete expansionunit of nine blocks it will be understood that a block E is always the central block. Centrally an inspection of Fi -1.

corner of a blocfi E,-i

block E is a .group of three projecting I brick 40 (Figs. 1 and 2) that, projecting brick 44 and46 above and below, hold intermediate soft'insulating) or refractorybrick 42 against which the ase 24 of block .E engages and is cushioned. Above and below block E with the bevels 23 and 27 with the in ,contact are blocks GL and GR, it being I remembered that GR is merely GL inverted.

Block GL is supported by its body 28 lying between a projecting masonry brick 44 and a and TR respectively, their recesses 30 and 36' facing inward to receive the NE and NW corner-portions of the bevel 23 on'block E,

and joining together the bevels 27 and 33 on the adjoimn blocks. and left of R (below E) are found TL and GL respectively, their recesses 36 and 30 also Similarly at the right.

facing inwardto receive the SE and 7 SVVf corner-portions respectively of the bevel 23 on block E, and also joining together the bevels 33 and 27- on the adjoiningblocks. It-

will be observed that a block TL is always found above a block GL and the rabbet 33 at the rear end of TL-is provided to receivea projecting masonry brick 48 thus permitting its under face to contact with the upper face of block GL. Likewise a block TB is always found below and fitting closely against the under face of a block GR, its rabbet 38 receiving a projecting masonry brick 50. As thus assembled the tongues and grooves fit into each other as'clearly shown by Fig. 4, permitting the various blocks to slide on each other in two directions when expanding under heat, but the various beveled surfaces hold the block E centrally, preserving the continuous expansion joint 12 about this block while yet permitting each block; to slide on the bevel as it expands.

bric 42 or otheryielding medium provided for this purpose.

Such expansion wed es the blocks Einward against the soft The dotted lines at the fouivoorners of the expansion unit illustratedby' Fig. 4 show the Lo n'der' of the recesses and bevels of the I hicks for receiving portions of the bases 24 of the four expansion blocks E located at'each of these corners as will be understood from n course 2 for example,'becomes GR below a block E located Thus GR at the 1 in course 1, and TR at the NW corner becomes TRat-the right of a block E in course 2. Similarly TL to. the left of a block E in course 4, and GL at the SW corner becomes -GL- above a TL at the SE corner becomes block E in course 5. Each expansion joint 12 therefore aids every other expansion joint 12 in accommodating the expansion of the blocks .at allfour sides of any block E.

' a fire wall spotted with expansion joints .each surrounded by freely slidable building blocks, any small portion of the wall may be submitted to heat of the most intense degree that will be met in the furnaces of superpower stations where a forced air feed is employed, and will be permitted to expand freely without regard to the tem erature of any other portion of the wall..- n like manner several or all of the expansion joints may function simultaneously. 'The'location and nature of the multitude of expansion joints effectually accommodates the expansion of any number of building blocks from all directions and over large or small areas of the fire wall.

Fig. illustrates a modified form of the central expansion member, denoted E, that may be used without loss of efiiciency. In this form the bevels are continuous from the the rear'face'61 to the front face 62. Accordingly each marginal block of the expansion unit must be modified by providing a longer beveled surface between their head and body portions. This longerbevel is shown at 63, in Fig. 5, for the blocks denoted G above and below an expansion member E. Theoblique surfaces 60 and 63 are in contact and the-member E is wedged inward, when the surrounding blocks expand under heat, against the soft insulating brick 64 which, as beffoil p,

projects from the surrounding brick masonry wall. 3 I

Fig. 9 is a further modification of the yielding support for the expansion blocks. Inthis figure the shape of the blocks E, G and T formin the fire wall is the same asiin Figs.

' 1 to 4, ut the masonry wall, immediately at the rear of each: block E, is built from two bricks 65and 66 shaped to form a cylindrical chamber or housing within the inner end of which is loosely seated a cylindrical block 67 .which maybe either ordinary fire brick or soft insulatingbrick- The sliding block '67 is backed by a coiled spring 68 the outer end.

of which is seated againsta collar .69 loose on a screw threaded rod 70 within the'coils of i the spring. The outer end of the rod 70 passes through a steel casing'72 on the outer face of the masonry wall and is normally held in the position shown by lock nuts 73 and 74.

- Figsi6 to 8 illustrate a mbdification of the masonry wall construction of Fig. '1to .provide lateral air-spaces behind the fire wall.

j What is claimed asnew, is

The construction is so changed that the air spaces behind each expansion block E (see 'Fig.1) are connected laterally instead of being isolated from each other by intervening solid brick.. In Fig. 6 the dotted lines 80 about each expansion block E indicate the area of the air-space backof these blocks. The communicating lateral passages formed by the modified construction are indicated by the dotted lines 82. It will be observed that the passag'es82 connect the lower portion of an air-space 80 with the upper portion of an air-space 80 in the .nextlower horizontal course of.blocks, so that a complete lateral air-space extends across the wall in a diagonal direction. Referring to Fig. 3 it is noted that the normal T block is rabbeted at the rear ported on the masonry wall by making the two bripks 84 and 86as' long as the bricks 40, 44 and 46 (Fig. 1) so thatthey will extend part way into the space formed by the two rabbets. The old masonrybricks 48 and 50 (Fig. 1) are shortened to the necessary extent. Thus a lateral passage 82 is formed through the two blocks TR and TL at the right and left (Fig. 6)' of each descending series, across the fire wall, of expansion blocks E. Each lateral passage 82 communicates, at

each end, withan air-space 80 (see Figs. 7 and 8). The series of lateral air-spaces extend diagonally across the fire wall, each in a series of steps, as shown b thedotted lines on Fig. 6 and by the sha 8 lines on Fig. 7. On

Figs. 6, 7 and'8 the blocks in the different horizontal courses have been" differentiated by exponents.

' Those skilled in the art will'understand that the building of the masonry wall 10 follows the usual construction of such walls except for the feature of the specially located sup orting shelves, hereinbefore described foru inding in the building blocks of the fire wa The invention is not limited to the use of the soft insulating br'ck 42 as a cushion for absorbing the inward t rust of the expansion blocks E while the ,wall is heating, since it is withinthe scope of the invention to utilize any suitable yielding element or device that is adequate for this purpose.

While it is preferred to employ the specific construction-and arrangement of arts shown and described it will be understo'od'that this construction is not essential except so far as specified in the claims, and may be changed or modified without departing from the broader ,features of the invention. p

relation to the hea 1. A furnace wall construction comprising a permanent masonry wall having a fire wall at its inner face comprised of refractory blocks all of which have the same dimensions at their heads except forcertain of saidblocks spotted over the wall whose heads are of locks V 2. A furnace wall construction comprising a permanent masonry wall having a fire wall at its inner face comprised of refractor blocks each havin a rectangularinner en face a minority 0 which are ofs Her dimensions at said end faces than the ajority thereof, said wall being erected from vertical rows of said blocks, and. each of said minority being wholly surrounded by. blocks of said majority and held with its inner end in spaced relation to said surrounding blocks.

3. A furnace wall construction comprisin a permanent masonry wall havin a fire wal at its inner face comprised of blocks each havin a rectangular inner end face a minority 0% which are of smaller dimensions at sald end-faces than the majority thereof, each of said minorit being wholly surrounded by blocks; of said majority and held withits inner end in spaced relation to said surrounding blocks, each of said minority blocks having a space between its rear face and the permanent wall, and means projecting from the permanent wall into said space for engagingjsaid rear faces.

4. A furnace-wall construction comprisin a permanent masonry wall having a fire wall at its inner face comprised of refractory blocks each havin a rectangular inner end face a. minority 0 which are of smaller disurrounded b held with its inner end in mensions at said end faces than the majority thereof, each of said minorit being wholly blocks of said majority and aced relation to said' surrounding blocks, an yielding means supported by said masonry wall againstwhich each of said minority blocks abuts.

5. A furnace wall construction comprising a permanent masonry wall in combmation with afire walLcomprised of rectan ular faced refractory blocks constructed an .arranged to provide a pluralityof rectangular expansionjoints over the wall face each of which joints surrounds the face of a single block, said blocks so surrounded by an expansion joint bein spaced from each other in the wall bothfhorizontally and vertically by a plurality of blocks in,abutting relation toeachother. e Y 6. A furnace wall construction comprising a permanent masonry wall in comb nation with a firewall comprisedof of said surrounding refractory faced: refractory blocks constructed andarf ranged to provide a plurality of rectangular expansion joints over the wall face each of which joints surrounds the face of asinglel block, and'means for holding' sa'id single-- blocks yieldingly in their centralized location within said expansion joints.

\ 7. A furnace wall construction comprising I 3a permanent masonry wall in'combination" with av fire wall comprised of rectangular faced refractoryblocks constructed and arranged to provide a plurality of rectangular expansion joints .over thewall face each of which joints surroundsthe face of a single block, and a yielding support at the rear of each single-block for maintaining their faces in the plane of the wall when the wall is cold but ermittingan inwardlmovement of said bloc s when the expansion joints close as the wall is heated.

8. A furnace wall 'construct ioncomprising a permanent masonry wall in combination with a fire wall comprised of rectan ular faced refractory blocks constructed an arranged to provide a pluralityof rectangular expansion. joints over the'wall'face each of which joints surrounds the'faceof a single block, said masonry wall having a yielding member built into its structlire at the rear of each of said single blocks. w

' 9. Afurnace wall construction comprising a permanent masonry wall in combination with a fire wall comprisedof rectangular] faced refractory blocks constructed and arranged to provide a plurality of rectangular.

expansion jointsioverthe wall face each of which joints surrounds the face of a single block, said masonry wall having a chamber at the rear of each of said single blocks and a plug engaging'the block backed by a, spring seated in each chamber. I

. 10. A firewall for furnaces erected from refracto blocks having rectangular inner ends, sai fir'ewall being spotted with a minority of blocks having inner ends of a smaller size than the majority providing rec,- tangular expansion joints about these smaller sized faces, said joints bein wholly surrounded by blocks of the majority that-are otherwise in contact.

refra'cto blocks having rectangular inner ends, sai fire wall being spotted with a mipredetermined initial relation to, each other 4 11. A fire wall for furnaceserected from rectangular ends,

characterized by a.v plura ity of rectangular expansion each comprising a plurality the margins of any one expansion unit are also included as marginalblocks of alltcontiguous expansion units. 7

13. In an expansion furnace wall comprised of refractory blocks having rectangular heads, an expansion unit composed of a central blockv and ei ht marginal blocks enclosing said "central lock but. spaced therefrom' along their inner edges, said, central block having an enlarged base joined to itsrectangular head by an inwardly facing bevel ateach side thereof, and said eight surrounding blocks each-havingan outwardly facing beveled portion adapted to engage said bevel of the central block. a

14. In an expansion furnace Wall comprised of refractory blocks. having rectangular heads, an expansion unit composed of a central block and eight marginal blocks. enclosing said central block but spaced therefrom along. their inner edges, said central block havin an enlarged base joined to its rectangular head by-an inwardly facing bevel at each side thereof, the four corner blocks of said surrounding blocks each having a corner recess shaped to receive the four corners of said beveled base, and the remaining four of said surrounding I wardly facing evel for engaging said inw'ardly facing bevel of the central block. 15. In-an expansion furnace wall comprised of refractory blocks having rectangular heads, an expansion unit consisting of eight blocks contacting with each other and so asl sembled that their heads together form a frame hollow at the center anda ninth block in said hollow center having a head smaller than said center which is' maintained out of contact with the heads ofsaid surrounding blocks.

16. In anexpansion furnace wall comprised ofrefractory blocks having rectangular heads, an expansion-unit consisting of-eight 1 blocks contacting with each other and so assembled that their heads together form a frame hollow at'the center and a ninth block in said hollow center having a head smaller than said center which is maintained out of contact with the heads of said, surrounding blocks but which is movable in and'out relative to the inner face of said wall. I

17. A furnace wall in which, a permanen masonrywall is combined' with a fire wall,

the latter comprising a plurality of horizontal courses of. re

vertical joints, the heads of certain blocks ractory blocks having aligned in each course being smaller than the re maining blocks of the course in both width and height and being so located with respect to the contiguous vertical and horizontal blocks as to provide expansion oints enblocks each having an outcourse with the relative smaller headedl'blocks int e several courses the latter comprising aplurality of horizontal courses of refractory blocks having aligned vertical joints, the heads of certain blocksin each course being smaller than the remaining blocks of the course in both. width and height and being so located with respectto the contiguous vertical and horizontal blocks as to provide expansion joints entirely separated rom' one another in'both the verticalrows and horizontal courses by the same surroundin them, the smaller: blocks being number of the full sized blocks.

A furnace wall in which a permaneri masonrywall is combined with a fire Wall, the latter com rising a plurality of horizontal courses of re ractory blocks aving aligned verticaljoints, the heads of ertain blocks in each course being smaller than the remaining blocks of the course in both width and height and being so located with respect to the contiguous vertical and horizontal blocks'as to provide expansion joints entirely surrounding them, the smaller blocks in each horizontal course being separate by at least four full sized blocks but in contiguous horizontal courses being ofl'set horizontally with respect to one another. 1

205A fire wall for-furnaces comprising. a plurality of refractory blocks having rec- 'tangula'r faces some of which blocks have smaller face dimensions than the remainder,-

the fire wall being spotted with said smaller blocks separated "from one another by the full sized blocks, and means for holding said smaller blbcks with the edges of their faces .in spaced relation to the face edges of the surroundin fblocks, whereby to prgvideexpansion joints entirely surrounding said smaller blocks separating them from the remaining blocks which are otherwise-in 0611- tact;

21; A furnace wall construction of the type in which a permanent wall is lined 'with a fire wall at its inner face, said fire wall being built of a plurality of courses of refractory blocks \having heads sodimensioned and shaped that the joints at the meeting faces thereof are continuous vertically'thus forming both vertical and horizontal rows of blocks} each of said courses having at regular spaced intervals aI-plurality of blocks each with a head of like shapebut of smaller di-.

mensions than the majority-of blocks in a ositions of said in! J bring so selected that the interval therebetween in the vertical rows is the same as that in the horizontal rows, and means for maintaining said smaller heads in spaced relation to all'surrounding larger headed blocks.

22. In an expansion fire wall construction erected from refractory blocks having rectangular heads a plurality of rectangular expansion units each comprising eight marginal blocks having rectangular heads in contact with each other and a single central block having a smaller rectangular head, interlocking means on the central and the marginal blocks for maintaining-a continuous expansion joint about said smaller headed block, and said expansion units being so built into the fire wall that each marginal block forms also a marginal part of a contiguous expansion unit of like construction.

23. In an'expansion fire wall construction erected from refractory blocks having rectangular heads a plurality of rectangular expansion units each comprising ei ht marginal blocks having rectangular hea s in con-' tact with each other and a single central block having a smaller rectangular head, interlocking means on the central and the marginal blocksfor maintaining a continuous expansion joint about said smaller headed block, said expansion units being so built into the fire wall that each marginal'block forms also amarginalpart of a contiguous expansion unit of like construction, and tongue and groove connections at certain of the meeting faces of said marginal blocks to permit relative sliding under expansion.

OSCAR NYGAARD. 

